This application is based upon and claims priority from Brazilian Patent Application No. PI 0004685-0 filed Oct. 5, 2000, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a method and device to stabilize the production of oil wells. The device may be used with an oil production pipe and is intended to overcome the harmful effects caused to the well by the flow of unstable mixtures produced by certain wells. More particularly, the present invention is preferably related to a device which is used with a flow pipe of an oil well equipped to produce oil by means of gas lift, and to a method for its use.
2. Description of the Related Art
Oil is usually found in accumulations under pressure in the subsoil, in porous and permeable sandstone known as reservoir stones, or else hydrocarbon producing rock formations. Wells are drilled from the surface to drain off such reservoirs so as to communicate the reservoir with processing facilities in the surface, which are assembled to collect and to treat the produced fluids.
The wells are bores which traverse several rocking formations. Usually a steel pipe is inserted into such bores, and is called a casing. At least one pipe of smaller diameter is inserted into such casing, through which fluids from the reservoir flow.
Oil is a complex mixture of heavy and light hydrocarbons, comprising from dry gas (methane) to heavy oil. Depending on the features of the reservoir, some components may appear in higher concentration than other. Other substances may also accompany the produced oil, such as water, carbonic gas, hydrogen sulfide gas, salts and sand, only to mention some examples.
Depending on the conditions of pressure and temperature, the constituents of the oil may be in the gaseous phase or in the liquid phase. Thus, it is concluded that the fluids that usually flow into an oil well may be defined as a multi-phase multi-component mixture.
The flow of the fluids into an oil well, from the reservoir to the surface, can occur as a consequence of the accumulated energy in the reservoir, that is, without the presence of an external source of energy which provokes such production. In such a case it is said that the production of the well is normally flowing, or else it is said that the well is producing by surge. When an external source of energy is used, e.g. a down hole pump, there is then what is called an artificial lift.
Among the various known artificial lift methods the continuous gas lift can be noted. In a usual configuration for this method, natural gas at high pressure is injected into an annulus formed between the casing and the pipe through which the production of fluids from the reservoir flows, which is also named the production string or tubing.
Valves known as gas lift valves are located at certain points along the tubing, which control the flow of gas flowing from the annulus to the interior of the tubing. The expansion of such pressurized gas provides the necessary additional energy to allow fluids from the reservoir to flow at a certain flow rate.
In some oil wells the flow of fluids into the tubing occurs in an unstable way, that is, there are variations of pressure and flow rate with time, which can even be harmful to the integrity of the well and its associated equipment.
There are in the technical literature many citations of severe cases in which unstable flows in oil wells cause a halt in production. Such instabilities are also known as xe2x80x9cheadingxe2x80x9d, as it is at the surface, at the well head, where they are more vigorously sensed, and such instability is able to occur in the tubing, in the annulus, or in both.
The phenomenon of the instability in the flow of multiphase mixtures is complex, and the causes for such instability are not totally understood. Generally, small disturbances give rise to great variations in the flow rates of the produced oil and the injected gas, as well as in the pressures. Many times such phenomenon is characterized by being cyclical.
In the article xe2x80x9cThese methods can eliminate or control annulus headingxe2x80x9d, by A. W. Grupping, C. W. F. Luca e F. D. Vermeulen (OilandGas Journal, Jul. 30, 1984, p. 192), the authors show that the unstable behavior of the flow in wells producing by means of continuous gas lift may frequently be attributable to the pressure oscillations in the annulus formed between the tubing and the casing. According to the authors, keeping the pressure constant causes the flow in such wells to stabilize.
The control of the injection of gas in wells equipped to produce by means of continuous gas lift is usually made by a gas choke valve, located at the surface, and by another valve located at a certain point in the tubing, which is the gas lift valve.
According to Grupping, Luca and Vermeulen, and some others, the ideal situation is to remove the control from the surface, allowing it to be made only by means of the gas lift valve. The authors also recommend that the gas lift valve be provided with an internal passage comprising a single orifice. However, this is not enough to keep the flow rate constant.
The conventional gas lift valves used to control the flow rate of injected gas in wells equipped to produce by means of continuous gas lift are not really valves, although they are designated as valves by the experts and by the manufacturers. Actually they are flow regulators equipped with a small disc provided with a round orifice having a certain diameter. The edges of the orifice are usually sharp or smoothly rounded.
Such gas lift xe2x80x9cvalvesxe2x80x9d are also provided with a check valve, located downstream of the orifice, so as to preclude an undesirable flow of oil from the tubing to the annulus to occur.
When a gas flows throughout a constriction, such as an orifice, and the pressure upstream of the orifice is kept constant, the flow rate of the flowing gas increases as the pressure downstream of the orifice decreases, until, for a certain upstream pressure known as critical pressure, the sonic speed of the constriction is achieved. From then on a decrease in the pressure downstream of the constriction will not cause the injected gas flow rate to raise.
Thus, there are two dynamic behaviors, or rates of flow, for a valve provided with an orifice. The first can be defined as a sub-critical rate of flow, in which a reduction in the downstream pressure causes a raise in the gas flow rate, and the second can be defined as a critical rate of flow, in which the gas flow rate is constant, independently of the downstream pressure (considering a constant upstream gas pressure).
In use, the pressure upstream of the orifice is basically the pressure of the injection gas existing in the annulus at the position where the gas lift valve is installed, and the pressure downstream of the orifice is basically the pressure of the flow of fluids into the tubing at the position where the gas lift valve is installed.
Thus, according to the above technical literature, in a situation where the flow is critical the use of the gas lift valve contributes to stabilize the flow into the well, as in this situation the flow rate of injection gas is constant (assuming that the pressure in the annulus is constant).
However, due to the irreversible losses of energy in a gas flow passing through such orifices, deriving basically from the heat, the friction and the sound coming from the extremely turbulent flow of gas under pressure passing through the orifice, there is a necessity for the pressure into the tubing being essentially less than 55% of the existing pressure in the annulus so as a critical flow is achieved.
Such differential of pressure is not usually found in most of the actual cases, and consequently the orifice valve operates in a sub-critical rate of flow, the variation in the gas flow caused by the variation of pressure into the tubing contributing to the instability of the flow in the well.
The Brazilian patent application PI9300292-0, commonly owned by the applicant and the description of which is herein incorporated for reference, contributed for the solution of the above problem by substituting a venturi for the orifice of sharp edges in the gas lift valves.
According to this document, the irreversible losses of energy in the injection gas flow are significantly smaller, and the increasing of the pressure in the diffusor of the venturi causes a critical flow to be achieved for a pressure in the tubing substantially smaller than 90% of the annulus pressure. Therefore, a critical flow is achieved more easily.
Consequently it is easier to keep constant the injection gas flow rate which, as previously mentioned, contributes to stabilize the flow into the tubing. Further, the smaller differential of pressure required by the gas lift valve with a venturi for injecting a certain flow of gas into the tubing provokes a more rational use of the available energy, thereby causing the costs for compressing gas to reduce (for the same oil flow rate), or increasing the income as a consequence of an increase in the production flow rate, be it for increasing the injection gas flow rate or injecting gas at a deeper position in the well.
However, in actual situations, the stabilization of the oil production is not always achieved simply using the gas lift valve with a venturi. Although a critical flow is achieved for tubing pressures higher than those in the situation where a conventional orifice is used, such tubing pressure is still low in severe instability situations.
The injection of gas by means of a gas lift valve with a venturi operating at a sub-critical rate of flow is even more harmful to the well than by means of gas lift valve with an orifice, and the instability can eventually augment. The sub-critical rate of flow in a gas lift valve with a venturi occurs in a range of 55% to 100% of the annulus pressure. In a gas lift valve with a venturi such range is reduced for 90% to 100%.
Thus, in a gas lift valve with a venturi operating at a sub-critical rate of flow the variation of pressure is about 4.5 times higher than a gas lift valve with an orifice operating at a sub-critical rate of flow. Such features of the gas lift valves with a venturi also makes it difficult to use such valves to inject gas at a deeper location, due to the existing uncertainty for calculations in a multiphase flow.
A mistake in the calculation can result in positioning the gas lift valve with a venturi at a location where the injection occurs in a sub-critical rate of flow (highly undesirable) or is even not possible (where the tubing pressure is higher than the annulus pressure). Thus, the use of a gas lift valve with a venturi is not the ultimate solution for all the cases where the well produces with instability.
There is then a need for a new solution to overcome the problem of stabilizing the production of an oil well, in particular in oil wells producing by means of continuous gas lift. Further, there is a need for a solution which enables the injection of gas at a deeper point in oil wells which produce by means of continuous gas lift.
The present invention relates to a method and device to stabilize the production of oil wells, the device intended to be inserted into the tubing of an oil well, which usually comprises:
a wellhead;
casing;
a tubing inserted into the casing;
a packer inserted and locked into the casing and connected to the tubing next to an oil reservoir, so as to create two discrete regions:
a lower chamber, extending downwardly from the packer to the reservoir; and
an upper chamber, or annulus, extending upwardly from the packer to the wellhead.
In a first aspect the invention provides an oil well comprising:
a tubing for carrying fluids coming from a reservoir to the surface;
a body inserted in the tubing, the body comprising:
a first portion that progressively reduces, in a direction of fluid flow in said tubing, a cross-sectional flow area for the passage of fluid;
a second portion, disposed downstream of said first portion with respect to the direction of fluid flow in said tubing, that defines a substantially constant cross-sectional flow area for the passage of fluid, said constant cross-sectional area being smaller than an unobstructed interior cross-sectional area of said tubing; and
a third portion, disposed downstream of said second portion, that progressively increases, in the direction of fluid flow in said tubing, a cross-sectional flow area for the passage of fluid.
In a second aspect the invention provides a body to stabilize the production of oil wells when provided to a tubing for carrying fluids coming from a reservoir; said body comprising:
a first portion which progressively increases in cross sectional area from a distal end thereof causing a progressive decrease, in a direction of fluid flow from said reservoir, in a cross-sectional area available for the passage of fluid from said reservoir when said body is inserted inside said tubing;
a second portion disposed adjacent to said first portion which has a substantially constant cross-sectional area defining a substantially constant cross-sectional area available for the passage of fluid when said body is inserted inside said tubing, said constant area being smaller than an unobstructed interior cross-sectional area of said tubing; and
a third portion disposed adjacent to said second portion which progressively decreases in cross-sectional area from a distal end thereof causing a progressive increase, in the direction of fluid flow, in a cross-sectional area available for the passage of fluid when said body is inserted inside said tubing.
In a third aspect the invention provides a device to stabilize the production of oil wells, said device comprising:
a body to stabilize the production of oil wells when provided to a tubing for conducting fluids coming from a reservoir; said body comprising:
a first portion which progressively increases in cross-sectional area from a distal end thereof causing a progressive decrease, in a direction of fluid flow from said reservoir, in cross-sectional area available for the passage of fluid when said body is inserted inside said tubing;
a second portion disposed adjacent to said first portion which has a substantially constant cross-sectional area defining a substantially constant cross-sectional area available for the passage of fluid when said body is inserted inside said tubing, said constant area being smaller than an unobstructed interior cross-sectional area of said tubing;
a third portion disposed adjacent to said second portion which progressively decreases in cross-sectional area from a distal end thereof causing a progressive increase, in the direction of fluid flow, in a cross-sectional area available for the passage of fluid when said body is inserted inside said tubing; and
a nipple tubing surrounding, and attached to, said body, said nipple tubing being insertable into said tubing of an oil well.
In a fourth aspect the invention provides a method to stabilize the production of oil wells comprising tubing for carrying to the surface the fluids coming from a reservoir, the method comprising:
inserting into said tubing a device comprising:
a first portion that progressively reduces, in a direction of fluid flow, a cross-sectional flow area for the passage of fluid flowing from said reservoir;
a second portion, disposed vertically above said first portion, defining a substantially constant cross-sectional flow area for the passage of fluid coming from said reservoir and smaller than an unobstructed interior cross-sectional area of said tubing;
a third portion, disposed vertically above said second portion, defining a progressively increasing, in a direction of fluid flow, cross-sectional area for the passage of fluid coming from said reservoir, until said cross-sectional area for the passage of fluid is equal to an unobstructed interior cross-sectional area of said tubing;
allowing fluids from said reservoir to flow towards said surface past said device, whereby said flow is accelerated when it passes said first portion, and consequently the flow pressure is decreased, said flow then, passing said second portion, and then passing said third portion, where said flow is decelerated, and consequently the flow pressure is increased, the above sequence causing a stabilization of said flow.
If the oil well is equipped to produce by means of continuous gas lift, a gas lift mandrel should be connected to the tubing and a gas lift valve should be connected to the gas lift mandrel. Injection gas at a high pressure should be injected at the wellhead in the annulus between the casing and the tubing of the oil well.
The gas lift valve should be provided with at least one port through which the high pressure injection gas of the annulus flows towards the interior of the tubing, and the device to stabilize the production must be inserted into the tubing with its medium portion located in front of the point where the high pressure injection gas is injected into the tubing.